Paper mills have for many years made extensive use, for the cleaning of paper making stock, of screening apparatus embodying a cylindrical perforated screening member defining inlet and accepts chambers on the opposite sides thereof in a closed housing and including a rotor member which operates in one of the chambers to keep the screening perforations open and free from solid material tending to cling to the screening surface. Commonly, the stock or furnish is delivered to the inlet chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collected and discharged from its opposite end.
The assignee of this invention has manufactured and sold many such screens in accordance with a series of U.S. patents, commencing with U.S. Pat. Nos. 2,347,716, to Staege and followed by 2,835,173; to Martindale, 3,849,302 and 4,105,543 to Seifert and 4,155,841 and 4,383,918 to Chupka-Seifert. Starting with the construction shown in the Martindale patent, all such screens manufactured and sold by applicant's assignee have been characterized by a rotor comprising bars or vanes of airfoil section moving in closely spaced but non-contacting relation with the surface of the screening cylinder for the purpose of creating alternating positive and negative pressure waves effective on the perforations in the screening cylinder to prevent plugging thereof.
The art has experimented widely with detailed variations in screens of the above type, including variations in the vane shape and other forms of rotor, and also in the size, configuration, and spacing of the perforations in the screening cylinder. Thus in the era of the Staege patent in the mid-1940's, the screening cylinder was fabricated from steel plate with multiple uniformly cylindrical drilled perforations. When this drilled plate was rolled into a cylinder, a natural result of the rolling operation was to effect some constriction and expansion, respectively, of the inner and outer ends of the drilled holes which were on the inlet (inside) and accepts (outside) surfaces of the resulting screening cylinder. This led in due course to the practice of relieving the accepts (discharge) end of each cylindrical perforation by a conical bore or countersunk portion to minimize the possibility of plugging.
In more recent years, the trade has been offered pressure screens generally of the above type wherein the perforations in the screening cylinder are elongated slots rather than round holes, typical such constructions being shown in U.S. Pat. Nos. 3,617,008, to Lamort, 3,581,903, to Holz, and the above-noted Seifert '302 and Chupka-Seifert patents. Both Lamort and Holz show slotted screening cylinders wherein the slots have parallel sided portions on the inlet side of the cylinder, but on the accepts side, each slot has widely diverging side walls. Similarly in Chupka-Seifert, the screening cylinder is fabricated from wire of triangular section with the base of the triangle on the inlet side of the cylinder.
This practice of providing relief on the accepts side of the perforations in the screening cylinder of a pressure screen was thus followed consistently in the paper industry since it was first introduced. The art demonstrated a conviction that this practice was necessary to minimize the possibility that the perforations would be plugged by the fiber and/or contaminant particles unless their minimum dimension is at their inlet ends. Indeed, this conviction became so firm that all of the claims of the Holz patent include limitations to a "boat-shaped" configuration of the screening slots on the accepts side of the screening cylinder.
Another patent issued to the assignee of this invention, Lehman U.S. Pat. No. 4,276,159, disclosed the discovery that the prior art had been wrong in its conviction that the screening perforations in the screening cylinder of a pressure screen needed to be relieved on the accepts side of the cylinder. More specifically, that patent disclosed that not only will such screens operate without plugging when the screening perforations are not relieved on the accepts side of the cylinder, but most unexpectedly, significantly improved results are obtainable when the perforations are relieved on the inlet side of the cylinder, e.g. in the same manner and to the same extent has had commonly been done on the accepts side of the cylinders of the prior art.
A recent modification of the Lehman patent screen is disclosed in Lampenius U.S. Pat. No. 4,529,520, wherein the inlet side of a screen cylinder is provided with axially extending grooves of trapezoidal section, and cylindrical screening holes are drilled in the bottoms of these grooves. The feature claimed in the Lampenius patent is that the downstream side of each groove is at a relatively oblique angle to the flow direction of the stock therepast, preferably an angle of approximately 30.degree., and in this respect, Lampenius closely followed the teaching in Fredriksson Canadian Patent No. 972,322, wherein screening perforations are shown on the upstream side of grooves or are provided with a countersunk relief on their downstream sides. The emphasis in both of those patents is on the desirability of reducing the target area of the inlet side of each screening perforation which is "seen" by the fibers in the flow therepast.